The instant invention is related to a method of making microemulsion blends having an average particle size of from about 0.001 microns to about 0.05 microns whereby the blend comprises at least one low amino content silicone and a surfactant having a high phase inversion temperature. The instant invention is further related to personal care products comprising said microemulsion.
Microemulsions containing silicone fluids have been found to be useful in a variety, of personal care products. As defined herein, the term "microemulsions" refers to transparent mechanically and thermally stable systems comprising small droplets having a mean or average particle diameter usually not more than 0.05 microns in diameter, preferably not more than 0.040 microns in diameter and most preferably not more than 0.025 microns in diameter. The small size of the droplets imparts a high degree of transparency to the emulsion.
The use of microemulsions is known in the art, see for example U.S. Pat. No. 4,797,272 (Linnet al.) and U.S. Pat. No. 4,620,878 (Gee). U.S. Pat. No. 4,797,272 to Linnet al. discloses water-in-oil microemulsion compositions having a mean droplet size ranging from about 0.001 microns to about 0.200 microns. U.S. Pat. No. 4,620,878 to Gee discloses a polyorganosiloxane emulsion that contains a polyorganosiloxane containing at least one polar radical such as an amino or ammonium radical attached to the silicon of the siloxane by Si--C or Si--O--C bonds or at least one silanol radical and at least one surfactant that is insoluble in the polyorganosiloxane. Water is added forming a translucent oil concentrate. The translucent oil concentrate is then rapidly dispersed in water to prepare emulsions with fairly low particle sizes. A drawback of Gee's teachings is that the oil concentrate must be diluted with very large quantities of water such that the final emulsion rarely contains more than about 5 wt. % silicone solids. The emulsions prepared by Gee typically have an average particle size of less than 0.14 microns.
Microemulsions of volatile silicones are taught in the art, for example U.S. Pat. No. 4,782,095 and U.S. Pat. No. 4,801,447, however these microemulsions have required large amounts of surfactants. The high levels of surfactants required in the prior art applications are detrimental in many applications.
Chrobaczek and Tschida in U.S. Pat. 5,057,572 teach the preparation of an aminoalkyl substituted polysiloxane where the silicone fluid, a water-soluble emulsifier, in contrast to Gee, water and an acid are combined and heated to 50.degree. C. The necessity, for a specific sequence of process steps, such as order of addition, is not taught by Chrobaczek. While Chrobaczek teaches this procedure is applicable to silicone fluids with an amino content of 0.1 meq./gr., in practice microemulsions result only when the amino content is above a threshold of about 0.12 to 0.14 meq./gr. Below this threshold level the particle size of the emulsion is such that the emulsion is hazy, and therefore not a true microemulsion, true microemulsions possess optical transparency to a greater or lesser degree, as measured by an ASTM haze number of less than about 150.
Breneman et al. in U.S. Pat. No. 5,234,495 teach the preparation of microemulsions through a process utilizing the blending of an organo modified polysiloxane, e.g. an aminofunctional polysiloxane, an organo modified polysiloxane emulsifier, water, and an alkaline metal salt. Heating such a blend above the cloud point of the mixture and simultaneously subjecting the mixture to high shear mixing produces a liquid phase that can be cooled to form a microemulsion.
Microemulsions of aminofunctional silicones, particularly high viscosity aminofunctional silicone fluids or gums, provide beneficial results when used in personal care product formulations. It continues to be desirable to provide alternative or improved methods for preparing microemulsions of small average particle size.